Puoi fare così:
codice:
<head>
<style type="text/css">
.nero {color:black;}
.rosso{color:red;}
</style>

<script language="Javascript">
function cambia(){
   document.Newsletter.f_mail.className="red";
}
</script> 
</head>

...
<input class="nero" name=f_mail type=text style="Courier New" value="immetti E-Mail" size="10" onClick="cambia();">